1. Field of the Invention
The subject matter of the invention is directed to a process for the production of hydroxylamine disulfonate diammonium salt (DS) and the effective reduction of NO.sub.x emissions from the process.
2. Description of Related Art
Hydroxylamine is widely useful in the transformation of organic compounds to derivatives, which in turn may be intermediates in pharmaceutical or other industrial synthesis of complex molecules. An important use of hydroxylamine is captively in the synthesis of caprolactam, the raw material for Nylon 6.
The classical method for the production of hydroxylamine is that ascribed to Raschig. An important commercial process consists of the reduction of ammonium nitrite with ammonium bisulfite and sulfur dioxide to give hydroxylamine disulfornate diammonium salt (DS). Upon hydrolysis, DS gives hydroxylamine sulfate, an intermediate in the production of caprolactam. The overall reaction for production of DS is: ##STR1## where NH.sub.4 HSO.sub.3 is produced from SO.sub.2 and a basic ammonium compound: EQU NH.sub.4 OH+SO.sub.2 .fwdarw.NH.sub.4 HSO.sub.3. (2)
In commercial practice, the reaction is conducted in a packed tower or column where a solution of the products is recirculated, an aqueous nitrite solution is fed to the recirculating product stream, and supplemental trim ammonia, such as ammonium hydroxide or ammonium carbonate is added in an amount to satisfy the stoichiometric requirements NO.sub.2 -/NH.sub.4 +=1/2, and SO.sub.2 gas, obtained for example by combustion of sulfur with air, is fed at the bottom of the packed column and is vigorously contacted with the liquid in the packed section of the tower.
The amount of SO.sub.2 fed to the tower should also satisfy the requirements of equation 1) above and the three reactants should therefore be fed in the molar proportions NO.sub.2 -/NH.sub.4 +/SO.sub.2 =1/2/2.
The reaction is complex, with the possibility of many side reactions which have been discussed authoritatively in a review by F. Seel, Fortschr. Chem. Forsch. 4, 301-332 (1963). Side reactions do occur and as a result NO.sub.x gases, predominantly NO, are released. Yields of DS are improved and the extent to which side reactions occur and NO.sub.x are formed is decreased when the reaction is carried out at lower temperatures, but even at 0.degree. C., a temperature expensive to maintain, NO.sub.x emissions primarily NO, still constitute up to 1500 volume ppm in the vent gas.
Addition of NO.sub.2 in a process for production of ammonium nitrite to improve yield and to reduce emission of nitrogen oxides is disclosed in Balint et al., U.S. Pat. No. 4,233,279, Nov. 11, 1980, also in Hertzog U.S. Pat. No. 4,045,540, Aug. 30, 1977, but there is no suggestion for addition of NO.sub.2 in the manner of this invention.
Abating NO.sub.x from the vent gases in the system of this invention by absorbing the gases in an alkaline solution is not satisfactory since NO does not react with alkali.
Converting NO to NO.sub.2 via the reaction 2NO+O.sub.2 .fwdarw.NO.sub.2 with O.sub.2 present in the mixture would require impractically long residence times, because of the low concentrations of NO present and the slow rate of the reaction.
It is desirable, therefore, to develop an effective method for abatement of NO.sub.x emissions from the process of production of DS.